Halve avocados lengthwise, twisting gently to separate halves. Remove seeds and peel avocado halves. Then mash the avocado meat or blend a few seconds in an electric blender. Mix with sour cream, onion powder, seasonings, 1 diced tomato and lemon juice. Taste the guacamole to see if you need to add more seasoning. Chill thoroughly. Sprinkle other tomato on top and serve with tortilla chips.